Hitherto, as low thermal expansion ceramic sintered bodies are known aluminum titanate, lithium alumino-silicate-based ceramics such as eucryptite, β-spondumene, and petalite, and magnesium alumino-silicate-based ceramics such as cordierite.
Though the aluminum titanate and the lithium alumino-silicate-based ceramics have a small thermal expansion coefficient, they have a small Young's modulus so that they are liable to deform by an external force or gravity. Accordingly, their application to precision machine parts or optical instrument parts in which dimensional changes or shape changes are disliked.
On the other hand, the cordierite has hitherto been applied as a low thermal expansion ceramic sintered body to filters, honeycombs, and refractories. However, this is a porous body, and its Young's modulus is low as from about 70 to 90 Gpa. Further, its thermal expansion coefficient is about 0.5 ppm/K, and it cannot be said that this value is sufficiently small.
Hitherto, in order to obtain a minute cordierite having a small thermal coefficient, there is known a method for making a petalite phase or a β-spodumene phase coexistent (see JP-A-11-209171 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). However, according to this method, cordierite sintered bodies having a sufficiently small thermal coefficient are not obtained. Further, there is known a technology in which a rare earth element is added in order to obtain a cordierite sintered body having a small porosity and a small thermal expansion coefficient (see JP-A-10-53460). However, even in this case, it cannot be said that the thermal expansion coefficient is sufficiently small.